In the field related to medical images, an image is captured by using a radioactive photographic film made of silver halide photosensitive material. In addition to this method, there is a method of obtaining radiographic image information using a photostimulable phosphor plate, without using a radiographic film.
Further, there is widespread use of the radiographic image input apparatus such as an X-ray computerized tomography (X-ray CT apparatus) and magnetic resonance imaging apparatus (MRI apparatus) other than the plain radiograph.
When such a medical image is used for diagnosis, in addition to the transparent recording medium placed on an illumination apparatus such as a viewing box (schaukasten), image information is often recorded on a reflective type recording medium and is examined in the form of a hard copy.
As described above, there are various forms of input and output method, and this requires appropriate gradation processing to be applied to each image data. Further, depending on the target region, there is a difference in the range of signal value to be emphasized. This calls for appropriate gradation processing.
FIG. 12 is a characteristic diagram illustrating an example of a gamma conversion processing characteristic as a gradation conversion processing. When the gamma value is smaller, there is an increase in the contrast of the low brightness, and this avoids possible black crushing (black compression). In the region of high brightness, however, white crushing tends to occur. If the gamma value is greater, there is an increase in the contrast of the high brightness, and this avoids possible white crushing. In the region of low brightness, however, black crushing tends to occur.
As described above, in the process of gamma conversion using any one gamma value, gradation processing that covers the overall brightness region cannot be implemented in the conventional art.
As a method for image processing intended for the aforementioned application, a combination of a plurality of steps of gamma conversion processing is proposed in the technique disclosed in the following Patent Document 1.    Patent Document 1: Unexamined Japanese Patent Application Publication No. 2003-310602 (P.1, FIG. 1).